4.3 The do-while Loop

false

FIGURE 4.2 The do-while loop executes the loop body first, then checks the loop- continuation-condition to determine whether to continue or terminate the loop.

The loop body is executed first. Then the loop-continuation-condition is evaluated. If the evaluation is true, the loop body is executed again; if it is false, the do-while loop terminates. The difference between a while loop and a do-while loop is the order in which the loop-continuation-condition is evaluated and the loop body executed. The while loop and the do-while loop have equal expressive power. Sometimes one is a more convenient choice than the other. For example, you can rewrite the while loop in Listing 4.4 using a do-while loop, as shown in Listing 4.5:

LISTING 4.5 TestDoWhile.java

1 import java.util.Scanner;

2

3 public class TestDoWhile {

4/** Main method */

5 public static void main(String[] args) {

6int data;

7 int sum = 0;

8

9// Create a Scanner

10 Scanner input = new Scanner(System.in);

11

12// Keep reading data until the input is 0

13do {

14// Read the next data

15System.out.print(

16"Enter an int value (the program exits if the input is 0): ");

17data = input.nextInt();

18

19sum += data;

20} while (data != 0);

21

22System.out.println("The sum is " + sum);

23}

24}

Enter an int value (the program exits if the input is 0): 3

Enter an int value (the program exits if the input is 0): 5

Enter an int value (the program exits if the input is 0): 6

Enter an int value (the program exits if the input is 0): 0 The sum is 14

loop

end loop

Initial-Action

The flow chart of the statement is shown in Figure 4.3(b). The for loop initializes i to 0, then repeatedly executes the println statement and evaluates i++ while i is less than 100.

The initial-action, i = 0, initializes the control variable, i. The loop- continuation-condition, i < 100, is a Boolean expression. The expression is evaluated right after the initialization and at the beginning of each iteration. If this condition is true, the loop body is executed. If it is false, the loop terminates and the program control turns to the line following the loop.

The action-after-each-iteration, i++, is a statement that adjusts the control variable. This statement is executed after each iteration. It increments the control variable. Eventually, the value of the control variable should force the loop-continuation-condition to become false. Otherwise the loop is infinite.

The loop control variable can be declared and initialized in the for loop. Here is an example:

for (int i = 0 ; i < 100; i++) { System.out.println("Welcome to Java!");

}

If there is only one statement in the loop body, as in this example, the braces can be omitted.

Tip

The control variable must be declared inside the control structure of the loop or before the loop. If the loop control variable is used only in the loop, and not elsewhere, it is good programming practice to declare it in the initial-action of the for loop. If the variable is declared inside the loop control structure, it cannot be referenced outside the loop. In the preceding code, for example, you cannot reference i outside the for loop, because it is declared inside the for loop.

initial-action

action-after-each- iteration

omitting braces

declare control variable